The use of filters to reject undesired signals and pass a signal of interest is a basic building block for signal processing applications. Filters are generally classified as either fixed or adaptive. If an undesired signal is present having a known frequency and bandwidth, and if the center frequency of the undesired signal is in the same frequency band as the signal of interest, the undesired signal may be rejected using a fixed notch filter. Fixed notch filters have a constant (non-time varying) center frequency and bandwidth. In general, if the bandwidth of the notch filter is small compared to the bandwidth of the signal of interest, the notch filter does not significantly distort the signal of interest.
However, if either the frequency or bandwidth of the signal of interest is not known a-priori, then an adaptive filter may be employed to dynamically change its center frequency and bandwidth as a function of the interference environment. Adaptive filters generally require a reference signal that is used to compute their frequency response. The reference signal may be a training signal transmitted over a channel. Alternatively, a receiver may generate its own reference signal, for example using bit detections in a decision feedback equalizer. Both of these types of adaptive filters have difficulty rejecting unwanted signals that change rapidly in time or frequency. Typically, classical adaptive filters can only track very slow time varying signals because many samples of the interference environment are necessary in order to converge to a desired frequency response. For example, when training sequences are used, an adaptive filter updates its response whenever a new training sequence is received resulting in slow response times.
Adaptive filters which generate their own reference signal are continuously adapting but long averages of the interference environment are necessary in order to converge to a desired frequency response. Another problem with these types of adaptive filters is that if the interference environment is too bad or changes too rapidly, they are no longer able to generate a valid reference signal and do not converge on the desired frequency response.
Thus, what is needed are a method and apparatus for adaptively canceling unwanted signals in a dynamic interference environment. What is also needed are a method and apparatus that can track relatively fast changes in an interference environment. What is also needed are a method and apparatus that rejects short duration narrow band interference signals in the presence of a wideband signal of interest. What is also needed are a digital filter apparatus that cancels undesired signals that vary rapidly in time.